Hybrid type inflators used for inflating airbags contain an inert gas in a chamber. The inert gas is heated by a burning a pyrotechnic composition within a heater structure in the inflator. The pyrotechnic composition is ignited by an igniter to initiate a rapid burning of a gas generant in a heater system. The gas generant heats the inert gas such that sufficient pressure can be achieved prior to releasing inflation gas to an airbag to inflate the airbag.
U.S. Pat. No. 5,660,412 A discloses a hybrid inflator that uses of a primary heater and a bypass heater to achieve more rapid heating of the pressurized fluid in the cylindrical gas generator. U.S. Pat. No. 5,660,412 A teaches a first pyrotechnic composition as a primary source of heating and a second pyrotechnic composition having a faster rate of gas generation wherein portions of these products of combustion directly heat the inflation gas using a single initiator.
More typically multi stage hybrid inflators utilize two igniters or initiators to separately ignite two separate gas generants or pyrotechnic compositions. Hybrid inflators of this type are disclosed in US 2004/0100079 A1, U.S. Pat. No. 6,793,244 B1, US 2003/0001370 A1 and U.S. Pat. No. 6,488,310 B1. In each of these publications the gas generant is contained in a heater element that can be activated in multiple stages by using two separate igniters. The sequences of firing are made possible by time delay such that when one activates the initiators they can be activated singularly or in parallel or sequentially.
The use of two separate heaters in opposite ends of the cylindrical inflator with the central exhaust has been taught in U.S. Pat. No. 6,253,683 B1 and EP 1 405 775 A1. These early prior art cylindrical inflators with a central exhaust did not employ bypass enhancer heaters to pre-heat the inert gas.
In the event these inflators were activated and an out of position occupant was detected, a means to bypass the inflation of the airbag to slow or stop the deployment of the airbag is desirable. To provide such a bypass feature mechanisms such as openings or hinged doors in the airbag module housing have been utilized as is shown and discussed in U.S. Pat. No. 6,971,671 B2 entitled “Active Venting Apparatus and Method For Airbag Systems”. In that prior art patent a pyrotechnic composition is attached to a membrane on either the housing or the airbag to rapidly create a large vent opening or exit port to allow inflation gasses to escape from the cushion. All of these systems required additional wires, initiators and components to be added to the housing module or airbag increasing the complexity and the cost of the device.
It is therefore desirable to achieve this rapid venting capability in a more efficient, far less complex manner and clearly in a more reliable fashion.